Panoramic radiographic images, sometimes also known as orthopantomographies, are well known in the dental field: they are two-dimensional radiographic images of a predefined curved plan approximating patient jaws; such image identifies a limited layer where anatomical structures are focused, outside which anatomical structures are blurred.
The main characteristics of a panoramic image are:                the thickness of the blurred area around the focused layer;        the acquisition angle, that is the angle defined between a direction of acquisition and a direction orthogonal to focused layer.        
These characteristics vary from pixel to pixel of the panoramic image itself.
A panoramic image is normally produced using a known specific panoramic apparatus, similar to that described e.g. in the Spartiotis EP2223653 patent, which exposes a patient's head to the X-rays emitted by an X-ray emitter. The rays are received by an X-ray sensor, provided with a matrix of receivers (or pixels) of few columns in width, which is opposed to the emitter. The sensor forms a partial radiographic image of the patient's head exposed to X-rays.
Moving the X-ray transmitter and receiver along a predefined trajectory, the complete panoramic image is produced acquiring a plurality of the partial radiographic images along the trajectory, and summing the partial images, which are superimposed and horizontally shifted by a suitable distance.
Alternatively, producing a panoramic image, which is per se a two-dimensional image, is also possible by processing 3D volumetric data obtained with a CBCT (Cone Beam Computerized Tomography) apparatus.
In this case a sensor having few columns in width is simulated, through which the partial image is acquired. The same acquisition method is virtually repeated, shifting the X-ray sensor and emitter on the predefined trajectory used in the above-described panoramic apparatus. The difference lies in the fact that each partial image is virtually calculated starting from 3D volumetric data expressed as voxels.
Such a method is known from the Ulrici, et al EP 2254475 patent, where a tomographic blurring method of an orthopantomograph is simulated. The panoramic image is calculated from the 3D volume previously acquired, corresponding each point of the panoramic image to a voxel of the 3D acquired volume which is selected as focused.
A problem of the panoramic images acquired both through a panoramic apparatus and through simulation starting from a 3D volume as in the Ulrici et al patent EP 2254475 arises from the fact that the panoramic image to be acquired lies on a curve.
From this the fact that in each partial image acquired by the (real or virtual) receiving sensor is only a column, typically the central column of such sensor, the sensor will correctly focus the desired point on that curve which follows. The other columns will have a positioning error which, when the sum of partial images is performed, leads to a slight blurring even in the curve itself which is to be focused.
Another problem is that the main features of an obtained panoramic image are connected, for each pixel, to those of the adjacent pixels, in that partial images are summed up superimposed to each other.
An alternative method to process 3D volumetric data to obtain a two-dimensional image “similar” to a panoramic image, as will be explained in the following, is known from the Pasini EP1973075 patent and Imaging Sciences International the Shi, et al U.S. Pat. No. 8,325,874.
The Pasini EP1973075 patent describes a processing method which extracts a layer of a 3D volume, having a defined thickness, laying between two different curves, one more internal and one more external to patient's head, and calculates an absorption in a direction orthogonal to a curve which is central between the external and the internal curve. According to this processing method there is no blurring, and therefore the two-dimensional image obtainable is “similar” to a panoramic image, in the sense that the image is “cleaner” with respect to a traditional panoramic image, in that some anatomical structures are not visible.